A long-term annual water balance analysis of the Araçuaí River Basin, Brazil

J. Geogr. Sci. 2010, 20(6): 938-946 DOI: 10.1007/s11442-010-0822-5

© 2010 Science China Press Springer-Verlag

Received: 2010-01-17 Accepted: 2010-03-10 Author: Leite Mariangela Garcia Praça, e-mail: [email protected] *Corresponding author: Fujaco Maria Augusta Gonçalves, Ph.D, e-mail: [email protected]

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A long-term annual water balance analysis of the Araçuaí River Basin, Brazil

LEITE Mariangela Garcia Praça, *FUJACO Maria Augusta Gonçalves Federal University of Ouro Preto, Geology Department, Campos Morro do Cruzeiro, 35400-000 Ouro Preto, Minas Gerais, Brazil

Abstract: Semi-arid and arid areas exhibit great temporal variability in water availability. In some of these regions, a one- or two-day rainfall is followed by intervening dry periods of variable length. In recent decades, many rivers of the semi-arid portion of the Jequitinhonha Basin have been undergoing a progressive discharge reduction, mainly of their base flow, the Araçuaí River is one of them. In order to understand this transformation, a long-term analysis of the annual water balance of the Araçuaí River Basin was performed, the results of which are presented herein. Satellite images, hydrometeorological and river discharge data were analyzed; mean and variance tests were conducted to determine temporal homogeneity. Historical pluviometric data analysis shows no corresponding precipitation reduction and temperature undergoes only a slight increase in the same period. On the other hand, evaporation is extremely high, higher than precipitation during most of the year, leaving al-most no water for infiltration (aquifer recharge) and runoff. Furthermore, the Araçuaí head-waters of its tributaries are now occupied by a monoculture, Eucaliptus sp., used for paper production. Because of the decreased fluvial discharges, its lowlands, usually used for agri-culture and pasture, are abandoned and partially eroded.

Keywords: river discharge; evaporation; Jequitinhonha Basin

1 Introduction

Society depends on the integrity of natural systems to provide goods and services for pro-duction and consumption. In particular, the goods and services generated by the movement of water through the landscape are assumed to make significant contributions to the liveli-hoods of rural communities. Arid and semi-arid regions exhibit great temporal variability in water availability and vegetation-soil water feedbacks. This is due to: shifting climate re-gimes over centuries and decades, inter-annual variations in weather patterns, seasonal pre-cipitation differences, and within-season precipitation variability as regards frequency and magnitude.

The movement of water through the continuum of soil, vegetation, and atmosphere is an important process. In arid and semi-arid regions, discrete rainfall pulses are separated by

LEITE et al.: A long-term annual water balance analysis of the Araçuaí River Basin, Brazil 939

intervening dry periods of variable lengths (i.e., interpulses). This situation results in water availability fluctuation, limiting plant production and controlling other ecological processes. Although many systems are characterized by wet and dry periods, the distinction is that these pulse/interpulse periods differ so dramatically in soil moisture that the biotic and abiotic functions associated with these periods also differ substantially, especially during the summer growing season (Austin et al., 2004; Huxman et al., 2004; Schwinning et al., 2004). Rainfall pulses can be large or small in magnitude and, in conjunction with temperature, wind, infiltration, surface flow, and evapotranspiration, determine the length of time that the soil is wet or dry. Patterns of wet and dry periods that occur at longer temporal scales inter-act with these finer scale pulse dynamics to influence water availability (Loik et al., 2004).

On the other hand, land use influences the hydrological catchment responses by either re-turning the rainfall to the atmosphere through evaporation and transpiration or directing it to aquifers and rivers (Hope et al., 2004). Understanding the water balance in relation to cli-mate and catchment characteristics provides an insight into the complex processes operating over a range of spatial and temporal scales. At the catchment scale, quantifying the effects of land-use change on water balance and predicting stream discharge into catchments are some of the major scientific challenges for hydrologists (Zhang et al., 2004; Zhang et al., 2008). Historical data analyses show that the Araçuaí River and its major tributaries experience an extended low water period from 1942–2002. The main objective of this research was to de-termine the influence of the hydrological cycle on this water reduction. In this study, the hydrological cycle is considered in its entirety. Links between rural livelihoods, land use and the goods and services provided by the evaporation and transpiration components of the hy-drological cycle (green water) are assessed through the analyses of rural livelihoods in the Araçuaí catchment, Jequitinhonha River Basin, Brazil. Results highlight the importance of water return to the atmosphere through evaporation and transpiration, and thus the impor-tance of land access and natural resource usage in disaggregated rural community livelihood strategies.

2 Materials and methods

2.1 Study site

The Araçuaí River Basin, located in the State of Minas Gerais, has a SW-NE general direc-tion (Figure 1) controlled by geological structures (Landa and Ferreira, 1995). The Araçuaí River is one the most important tributaries of the Jequitinhonha River, with three main tributaries: Itamarandiba River, Fanado River, Setúbal River and Gravatá River and as sec-ondary tributaries: Black Creek, Soledade Creek, Itanguá Creek, Sto. António Creek and Calhauzinho Creek, this last one, stands out for its high degree of pollution, coming from a dairy located near to the Araçuai city, which launches its waste directly to the river.

The regional climate is relief controlled oscillating between sub-humid and semi-arid. Two seasons can be defined: a wet and hot period (October to March); and a dry and cold period (April to September) (Figure 2).

The vegetal covering is mainly savannah, penetrated by forest galleries and bushes. For-merly, the vegetal formations were the rain forest, the deciduous forest and the savannah. During the last decades, these natural biomes have been destroyed by human activities, such

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Figure 1 Araçuaí basin location map

Figure 2 Histogram showing precipitation variations in Araçuai basin

as agriculture and cattle breeding. Both of them have caused an enormous impact on the original forest physiognomy (Leme Engenharia, 1987).

2.2 Landcover data collection and analyses

The spatial database consisted of land-cover maps derived from satellite images (Landsat – 2000) with a special resolution of 15 m. The Landsat classification, a digital elevation model

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derived from a digitized 1:500.000 topographic map and a geologic map (1:100.000) of the study area were also included in the GIS database. Hydrometeorological data

The hydrometeorological data has been analyzed to determine temporal homogeneity us-ing mean and variance tests. All the data was implemented on the GIS platform, and a series of precipitation maps were developed using the nearest neighbor routine method. The sta-tions used in this study are listed in Table 1.

Table 1 List of the stations used in this study (data: ANA, 2009; INMET, 2009)

Precipitation Data discharge Evaporation/Temperature

Araçuai Carbonita Araçuai Carbonita Araçuai Carbonita

Pega Turmalina Pega Turmalina

Berilo Berilo

Ponte Alta Ponte Alta

Senador Modestino Senador Modestino

Porto Santana Rio Preto Porto Santana

Setubal

Chapada

Minas Novas

3 Results and discussion

Satellite images show that intense deforestation occurred during the last century (Figure 3). Part of headwaters of the Araçuaí tributaries is now occupied by monocultures, especially Eucaliptus sp., used for paper production (Figure 3). The lowlands that were usually used

Figure 3 Satellite image from part of the Araçuaí basin showing deforestation and Eucaliptus sp. Monoculture

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for agriculture and pasture have been abandoned because of the decrease in fluvial discharge. This was done without vegetation restoration (Figure 2) and part of the land is eroding.

Data analyses showed that the total annual precipitation in the basin is approximately 783,7 mm (Figure 4a), distributed in two well-defined seasons (Figure 4b). Over 80% of the precipitation is concentrated during a few days (mean = 22 days/year) and over the months of November to March.

Figure 4 Histograms showing: a) total annual precipitation (mean = 783,7 mm; b) total monthly precipitation mean–Data: Araçuaí station 1919 to 2008

Spatial analysis of precipitation data shows a clear pattern, with main precipitation occur-

ring in the south (Figure 5). Contrasting with precipitation, the evaporation rate is greater from July to November

(Figure 6a). Evaporation data indicates there was a little increment during the last decade (Figure 6b), and this can be related to temperature rise (Figure 6c). The hydrological cycle deficit is enormous (Figures 7a and 7b). For the last 50 years, precipitation has been greater than evaporation in only four of them. As a consequence, fluvial discharge has been reduced (Figures 8a, 8b and 9). When analyzing only the dry season months (Figure 8b), this reduc-tion is clearer: from 50 to 100 m3/s in the 1940s to less than 50 m3/s during the last decade.

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Figure 5 Map showing the total mean precipitation spatial distribution

Figure 6 Graphics showing: a) monthly evaporation variation (1950–2008); b); total annual evaporation variation (1950–2008); c) total annual mean tem-perature variation (1923–2008). DATA: Araçuaí station

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Figure 7 Graphics showing: a) difference between monthly precipitation and evaporation; b) difference between total annual precipitation and evaporation – DATA: Araçuaí station, 1942–2008

4 Conclusions

Aiming for greater efficiency and reduced impacts on limited natural resources calls for in-tegrated planning. Water resource planning must be governed by land-use planning elabo-rated within a spatial plan, since land-use allocation controls water system demand and availability. This hydrologic availability depends on precipitation and evaporation, where precipitation is fixed by geography, but evaporation is dependent on land use. A semi-arid climate or steppe climate generally describes climatic regions that receive low annual rainfall (250–500 mm or 10–20 in). All Araçuaí River Basin characteristics (vegetation types, rivers patterns) allow classifying it as a semi-arid area. The reason for this is that even though its total annual precipitation is superior to 600 mm, evaporation is extremely high; as a consequence, almost all rainfall is returned to the atmosphere. The mean annual hydrologic deficit is around 1000 mm, implying that only a reduced amount of the water balance infil-trates or runs off the land. Furthermore, large abandoned areas occupied with the Eucaliptus

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Figure 8 Graphics showing: a) Araçuaí river’s total yearly discharge variation and b) Araçuaí river’s total monthly discharge (dry season) – DATA: Araçuaí station, 1942–2008

Figure 9 Graphics showing: Araçuaí river’s total yearly discharge variation and b) Araçuaí river’s annual total precipitation– DATA: Araçuaí station, 1942–2008

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sp. monoculture at the headwaters of its tributaries, associated with increasing lowland ero-sion, has the linked effect of increased runoff with a concomitant infiltration reduction and aquifer recharge. The result is continuous fluvial discharge decrease, leading to intermittent Araçuaí tributaries.

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